Hi,
I understand your requirement but it is standard behavior of system that PO number is allowed to save without line item number.
---- Important notes regarding the object link functionalities are:
Note Description
880077 Performance problem using object links
885977 Extension to note 867241 on object links
926060 Extension of note 885977 for object link Purchase Order
960202 Object Links Deleted While Adding New Objects
1002648 Wrong behaviour while using RFC_CHANGE_DOCUMENT_MASTER
1034682 Object links getting deleted while scrolling and switching
1050158 Documents links lost in the Notif. when clicked on Sales order
332046 Document Links in IW53
Rgds,
Nayeem.